vhg ... 1/2 right ... some old PC drives will only read - or + (but this almost always only applied to CD drives ... hardly ever DVD drives)
DVD + is optimised for media
DVD - is optimised for data
The file allocation table and sector sizes are the key
a data file data can be large or small ... but must be accurate the emphasis is on error correction ... not necessarily on speed of access.
although because a CD/DVD is usually written all at once
fragmentation is not generally an issue
Media on the other hand ... tends to be large, digital .... and rely on a steady stream of data which can be relatively error tolerant - the signal is decoded as it is being extracted ...and as it is being played so there is a double error "trap"
most domestic dvd TV recorders/players work to + .... quite a lot can't cope with - so mp3 despite being technically data .. is best recorded to + discs